Air-fuel ratio control apparatus of internal combustion engine

ABSTRACT

An air-fuel ratio control apparatus of an internal combustion engine includes: an exhaust gas purifying catalyst disposed in an exhaust passage of the internal combustion engine; fuel cutting means for cutting fuel to be supplied to the internal combustion engine at the time of deceleration of the internal combustion engine; and catalyst degradation suppressing means for suppressing degradation of the exhaust gas purifying catalyst by prohibiting operation of the fuel cutting means when it is determined that the degradation of the exhaust gas purifying catalyst advances, wherein the catalyst degradation suppressing means sets an operation permitting period, during which the operation of the fuel cutting means is permitted after completion of a fuel quantity increasing operation of the internal combustion engine.

This is a Divisional of application Ser. No. 11/002,489 filed on Dec. 3,2004. This application claims the benefit of Japanese Patent ApplicationNo. 2003-407449, filed Dec. 5, 2003. The entire disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

The present invention relates to an air-fuel ratio control apparatus ofan internal combustion engine.

There has been conventionally known an air-fuel ratio control apparatusof an internal combustion engine in which when fuel is increased inquantity in the internal combustion engine and an output from an oxygenconcentration sensor, which is disposed downstream from a catalystconverter, is rich, an air-fuel ratio is controlled to be a leanair-fuel ratio for a predetermined period of time, and thereafter, theair-fuel ratio is controlled to be returned to a theoretical air-fuelratio (see Japanese Patent Application Laid-Open (JP-A) No. S63-117139).In addition, the prior art relevant to the present invention isdisclosed in Patent JP-A Nos. S63-134835, H6-307271, S59-173533, andH2-188616.

The capacity of a catalyst in an exhaust gas purifying system mounted ona vehicle or the like has been increased in order to cope with thereinforcement of emission control. Therefore, there is a possibilitythat a conventional air-fuel ratio control cannot suppress generation ofa catalyst exhaust gas odor (specifically, an odor of hydrogen sulfide(H₂S)) after fuel quantity increasing operation of the internalcombustion engine, because a quantity of oxygen occluded in an exhaustgas purifying catalyst is small till deceleration or stoppage of thevehicle. In order to occlude the oxygen in quantity enough to suppressthe generation of the catalyst exhaust gas odor with respect to thecatalyst till the deceleration or stoppage of the internal combustionengine, for example, the air-fuel ratio is largely changed onto a leanside by cutting the fuel or the internal combustion engine is operatedin a lean air-fuel ratio for a long period of time. However, there maybe a problem of a miss fire in the internal combustion engine when theair-fuel ratio is largely changed onto the lean side, while there may bea problem of degradation of exhaust emission due to an increase inNO_(x) generation quantity during operation in a lean air-fuel ratio fora long period of time. Additionally, the degradation of the catalyst isintensified in the atmosphere in which the oxygen is excessively presentat a high temperature.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide an air-fuel ratio control apparatus of aninternal combustion engine, in which oxygen in quantity capable ofsuppressing generation of a catalyst exhaust gas odor till thedeceleration or stoppage of the internal combustion engine can besecurely occluded in an exhaust gas purifying catalyst while suppressingdegradation of the exhaust gas purifying catalyst.

In the first aspect of the present invention, there is provided anair-fuel ratio control apparatus of an internal combustion enginecomprising: an exhaust gas purifying catalyst disposed in an exhaustpassage of the internal combustion engine; fuel cutting means forcutting fuel to be supplied to the internal combustion engine at thetime of deceleration of the internal combustion engine; and catalystdegradation suppressing means for suppressing degradation of the exhaustgas purifying catalyst by prohibiting operation of the fuel cuttingmeans when it is determined that the degradation of the exhaust gaspurifying catalyst advances, wherein the catalyst degradationsuppressing means sets an operation permitting period, during which theoperation of the fuel cutting means is permitted after completion of afuel quantity increasing operation of the internal combustion engine.

According to the air-fuel ratio control apparatus of the first aspectaccording to the present invention, the catalyst degradation suppressingmeans sets the operation permitting period of the fuel cutting meansafter the fuel quantity increasing operation of the internal combustionengine, so that the fuel is cut so as to make the air-fuel ratio of theexhaust gas lean at the time of the deceleration of the internalcombustion engine. Consequently, it is possible to supply oxygen to theexhaust gas purifying catalyst in the reduced oxygen occlusion quantityby the fuel quantity increasing operation, thus occluding the oxygen inthe exhaust gas purifying catalyst till the stoppage of the internalcombustion engine.

In the first aspect of the air-fuel ratio control apparatus according tothe present invention, the catalyst degradation suppressing means maydetermine a completion timing of the operation permitting period basedon an integration quantity of intake air taken into the internalcombustion engine after the completion of the fuel quantity increasingoperation, or the catalyst degradation suppressing means may determine acompletion timing of the operation permitting period based on a lapse oftime after the completion of the fuel quantity increasing operation. Thequantity of oxygen occluded in the exhaust gas purifying catalyst can beestimated based on the integrating air intake quantity or the lapse oftime after the fuel quantity increasing operation. Thus, it is possibleto shorten the period, during which the atmosphere of the exhaust gaspurifying catalyst becomes lean, so as to suppress the degradation ofthe exhaust gas purifying catalyst by completing the operationpermitting period based on the integrating air intake quantity or thelapse of time after the fuel quantity increasing operation in theabove-described manner.

Furthermore, in the first aspect of the air-fuel ratio control apparatusaccording to the present invention, the catalyst degradation suppressingmeans may determine a completion timing of the operation permittingperiod based on an integration quantity of intake air taken into theinternal combustion engine after an air-fuel ratio of exhaust gasflowing into the exhaust gas purifying catalyst is changed to a leanair-fuel ratio, or the catalyst degradation suppressing means maydetermine a completion timing of the operation permitting period basedon a lapse of time after an air-fuel ratio of exhaust gas flowing intothe exhaust gas purifying catalyst is changed to a lean air-fuel ratio.Since no oxygen flows into the exhaust gas purifying catalyst when theair-fuel ratio is not lean even after the fuel quantity increasingoperation, no oxygen is occluded in the exhaust gas purifying catalyst.Thus, it is possible to more accurately determine whether the oxygen inquantity enough to suppress a catalyst exhaust gas odor is occluded inthe exhaust gas purifying catalyst by determining the completion timingbased on the integrating air intake quantity or the lapse of time afterthe air-fuel ratio of the exhaust gas flowing to the exhaust gaspurifying catalyst is changed to a lean air-fuel ratio in theabove-described manner.

The air-fuel ratio control apparatus of the first aspect according tothe present invention may further comprise air-fuel ratio controllingmeans for controlling an air-fuel ratio in the internal combustionengine, and the air-fuel ratio controlling means may set a lean controlperiod, during which the air-fuel ratio is controlled to be a leanair-fuel ratio only at the deceleration of the internal combustionengine, after the completion of the fuel quantity increasing operation.In this case, the air-fuel ratio is not controlled to be the leanair-fuel ratio except for the time of the deceleration of the internalcombustion engine, and the degradation of the exhaust gas purifyingcatalyst can be suppressed. In contrast, since the air-fuel ratio iscontrolled to be the lean air-fuel ratio at the time of the decelerationof the internal combustion engine, the oxygen is supplied during thedeceleration, so that the oxygen can be occluded in the exhaust gaspurifying catalyst till the stoppage of the internal combustion engine.

After the oxygen in quantity enough to suppress the catalyst exhaust gasodor is occluded in the exhaust gas purifying catalyst, the degradationof the exhaust gas purifying catalyst can further be suppressed unlessthe air-fuel ratio is controlled to be the lean air-fuel ratio even atthe time of the deceleration of the internal combustion engine. Thus, inthe first aspect of the air-fuel ratio control aspect according to thepresent invention, the air-fuel ratio controlling means may determine acompletion timing of the lean control period based on an integrationquantity of intake air taken into the internal combustion engine afterthe completion of the fuel quantity increasing operation, or theair-fuel ratio controlling means may determine a completion timing ofthe lean control period based on a lapse of time after the completion ofthe fuel quantity increasing operation.

Moreover, the air-fuel ratio controlling means may determine acompletion timing of the lean control period based on an integrationquantity of intake air taken into the internal combustion engine afteran air-fuel ratio of exhaust gas flowing into the exhaust gas purifyingcatalyst is changed to a lean air-fuel ratio, or the air-fuel ratiocontrolling means may determine a completion timing of the lean controlperiod based on a lapse of time after an air-fuel ratio of exhaust gasflowing into the exhaust gas purifying catalyst is changed to a leanair-fuel ratio. Thus, it is possible to more accurately estimate anoxygen occlusion quantity in the exhaust gas purifying catalyst by thedetermining the completion timing in the above-described manner.

In the first aspect of the air-fuel ratio control apparatus according tothe present invention, the air-fuel ratio controlling means may set thelean control period to be shorter as a stoichiometric operation period,during which the internal combustion engine is operated in astoichiometric air-fuel ratio after the fuel quantity increasingoperation of the internal combustion engine, is longer. The oxygen iscontained also in the exhaust gas to be exhausted during thestoichiometric operation of the internal combustion engine. Therefore,the oxygen is gradually occluded in the exhaust gas purifying catalystalso when the internal combustion engine is stoichiometrically operated.Consequently, when the period of the stoichiometric operation after thefuel quantity increasing operation is long, the lean control period canbe shortened. Thus, it is possible to suppress the degradation of theexhaust gas purifying catalyst.

In the first aspect of the air-fuel ratio control apparatus according tothe present invention, the air-fuel ratio controlling means may set thelean control period when a fuel cutting period, during which the fuelcutting means cuts the fuel, is shorter than a predetermined value,after the fuel quantity increasing operation of the internal combustionengine. During the fuel cutting period, the concentration of the oxygenin the exhaust gas is substantially equal to that of the air, so thatmuch oxygen can be occluded in the exhaust gas purifying catalyst.However, when the fuel cutting period is short, there is a possibilitythat the oxygen in quantity enough to suppress the catalyst exhaust gasodor is not occluded in the exhaust gas purifying catalyst. Thus, theoxygen can be sufficiently occluded in the exhaust gas purifyingcatalyst by setting the lean control period when it is determined thatthe fuel cutting period is short.

The air-fuel ratio control apparatus of the first aspect according tothe present invention may further comprises oxygen integration flow rateacquiring means for acquiring an integration flow rate of oxygen flowinginto the exhaust gas purifying catalyst, and the air-fuel ratiocontrolling means may set the lean control period when the oxygenintegration flow rate acquired by the oxygen integration flow rateacquiring means during the fuel cutting period is lower than apredetermined quantity. Thus, it can be more properly determined whetherthe lean control period is set based on the integration flow rate of theoxygen flowing into the exhaust gas purifying catalyst during the fuelcutting period in the above-described manner.

In the second aspect of the present invention, there is providedair-fuel ratio control apparatus of an internal combustion enginecomprising: a plurality of exhaust gas purifying catalysts disposed inan exhaust passage of the internal combustion engine; secondary airsupplying means for supplying secondary air to at least one portionbetween the plurality of exhaust gas purifying catalysts through asecondary air passage; valve means for switching connection to ordisconnection from the secondary air passage; valve controlling meansfor controlling operation of the valve means; and fuel cutting means forcutting fuel to be supplied to the internal combustion engine at thetime of deceleration of the internal combustion engine, wherein thevalve controlling means sets a supplying period, during which thesecondary air is supplied by switching the valve means to a connectionstate during a non-operating period of the fuel cutting means aftercompletion of a fuel quantity increasing operation of the internalcombustion engine.

According to the air-fuel ratio control apparatus of the second aspectaccording to the present invention, the secondary air is supplied by thevalve controlling means after the fuel quantity increasing operation, sothat the secondary air can be supplied to the exhaust gas purifyingcatalyst downstream from a position where the secondary air passage isconnected. Therefore, the oxygen in quantity enough to suppressgeneration of a catalyst exhaust gas odor can be occluded in the exhaustgas purifying catalyst on the downstream side. Furthermore, no secondaryair is supplied to the exhaust gas purifying catalyst upstream from theposition where the secondary air passage is connected, therebysuppressing the degradation of the exhaust gas purifying catalyst on theupstream side.

In the second air-fuel ratio control apparatus of the second aspectaccording to the present invention, the valve controlling means may setthe supplying period to be shorter as a fuel cutting period by the fuelcutting means after the completion of the fuel quantity increasingoperation is longer. The exhaust gas containing oxygen at substantiallythe same concentration as that of the air is supplied to the pluralityof exhaust gas purifying catalysts during the fuel cutting period.Therefore, the same effect as that produced when the air is supplied tothe plurality of exhaust gas purifying catalysts can be produced duringthe fuel cutting period. The supplying period, during which thesecondary air is supplied, can be further shortened as the fuel cuttingperiod is longer. In this manner, the period, during which theatmosphere of the exhaust gas purifying catalyst on the downstream sidebecomes lean, can be shortened by shortening the supplying period, thussuppressing the degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an internal combustion engine, to which anair-fuel ratio control apparatus according to the present invention isapplied, in the first embodiment;

FIG. 2 is a flowchart showing a catalyst degradation suppressing controlroutine executed by an ECU shown in FIG. 1;

FIG. 3 is a flowchart showing the first example of an air-fuel ratiocontrol routine executed by the ECU shown in FIG. 1;

FIG. 4 is a flowchart showing the second example of an air-fuel ratiocontrol routine executed by the ECU shown in FIG. 1;

FIG. 5 is a graph showing one example of the relationship between anintegration GaS and a determination integration value E;

FIG. 6 is a flowchart showing the third example of an air-fuel ratiocontrol routine executed by the ECU shown in FIG. 1;

FIG. 7 is a flowchart following FIG. 6;

FIG. 8 is a graph showing one example of the relationship between anintegration Ga and an oxygen integration flow rate;

FIG. 9 is a timing chart showing one example of changes in time ofoxygen occlusion quantities in catalysts and an output of an oxygenconcentration sensor when the control routine shown in FIG. 6 isexecuted;

FIG. 10 is a diagram showing an internal combustion engine, to which theair-fuel ratio control apparatus according to the present invention isapplied, in the second embodiment;

FIG. 11 is a flowchart showing a supplying valve control routineexecuted by an ECU shown in FIG. 10;

FIG. 12 is a graph showing one example of the relationship between afuel cutting period and a determination period G;

FIGS. 13A and 13B are diagrams showing modifications of the secondembodiment according to the present invention;

FIG. 14 is a flowchart showing a modification of the catalystdegradation suppressing control routine shown in FIG. 2;

FIG. 15 is a flowchart showing a modification of the air-fuel ratiocontrol routine shown in FIG. 3; and

FIG. 16 is a flowchart showing a modification of the air-fuel ratiocontrol routine shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a diagram showing an internal combustion engine, to which anair-fuel ratio controller according to the present invention is applied,in the first embodiment. An internal combustion engine 1 is providedwith cylinders 2 (four in FIG. 1). As well known, to the internalcombustion engine 1 are connected an intake passage 3 and an exhaustpassage 4. In the intake passage 3, there are provided an air filter 5for filtrating intake air, an air flow sensor 6 for outputting a signalaccording to an intake air quantity, and a throttle valve 7 foradjusting the intake air quantity. In the exhaust passage 4, there areprovided an air-fuel ratio sensor 8 for outputting a signal according toan air-fuel ratio of exhaust gas which is exhausted from the internalcombustion engine 1, an exhaust gas temperature sensor 9 for outputtinga signal according to a temperature of the exhaust gas, a start catalyst10, an oxygen concentration sensor (hereinafter simply referred to as“an O₂ sensor”) 11 for outputting a signal according to an oxygenconcentration in the exhaust gas, and a three way catalyst 12 serving asan exhaust gas purifying catalyst. The start catalyst 10 is provided forthe purpose of reduction of an exhaust quantity of hazardous substanceuntil the three way catalyst 12 is activated at the time of cold startof the internal combustion engine 1, and thus the start catalyst 10 alsoserves as an exhaust gas purifying catalyst. As the start catalyst 10,for example, a three way catalyst is used. These catalysts 10 and 12 canocclude oxygen therein. When carbon monoxide (CO) or hydrocarbon (HC) ispresent in the exhaust gas, CO or HC is oxidized by using the occludedoxygen, thereby purifying the exhaust gas. Otherwise, when an oxidecomponent such as NO_(x) is contained in the exhaust gas, the oxidecomponent is reduced, thereby purifying the exhaust gas.

The operating state of the internal combustion engine 1 is controlled byan engine control unit (hereinafter abbreviated as an “ECU”) 13. The ECU13 is configured as a computer including a microprocessor and peripheraldevices such as a ROM and a RAM required for operation of themicroprocessor in combination. The ECU 13 controls operation of a fuelinjection valve 14 disposed for each of the cylinders 2 with referenceto an output from, for example, the air-fuel ratio sensor 8 and the O₂sensor 11, and further, supplies a proper quantity of fuel to each ofthe cylinders 2 such that the air-fuel ratio of the exhaust gas becomesa target air-fuel ratio. In this manner, the ECU 13 functions asair-fuel ratio controlling means by controlling the operation of thefuel injection valve 14. Furthermore, the ECU 13 cuts the fuel to besupplied to the internal combustion engine 1 for the purpose ofreduction or the like of fuel consumption when an engine speed of theinternal combustion engine 1 is a predetermined engine speed (forexample, 1,000 rpm) or higher at the time of deceleration of theinternal combustion engine 1. Hereinafter, such fuel cut operation maybe abbreviated as a “F/C”. In this manner, the ECU 13 also functions asfuel cutting means by cutting the fuel to be supplied to the internalcombustion engine 1.

The degradation of each of the catalysts 10 and 12 advances if acatalyst temperature is high and oxygen is excessively contained in theexhaust gas by the F/C or the like. In view of this, the ECU 13 executesa catalyst degradation suppressing control routine shown in FIG. 2, andthen, prohibits the F/C so as to suppress the degradation of thecatalyst 10 or 12 if it is determined that the degradation of thecatalyst 10 or 12 advances. The control routine shown in FIG. 2 isexecuted repeatedly in a predetermined cycle during the operation of theinternal combustion engine 1. The ECU 13 serves as catalyst degradationsuppressing means by executing the control routine shown in FIG. 2.

In the control routine shown in FIG. 2, the ECU 13 first determines instep S11 whether an F/C condition at the time of the deceleration of theinternal combustion engine 1 is established. If it is determined thatF/C condition is not established, the current control routine is ended.In contrast, if it is determined that the F/C condition is established,the control routine is proceeded to step S12, where the ECU 13determines whether a precondition for execution of a degradationsuppressing control for the catalyst 10 or 12 is established. It isdetermined that the precondition for the degradation suppressing controlis established when, for example, the throttle valve 7 is normal, i.e.the throttle valve 7 has not been failed in its operation. In contrast,if it is determined that no precondition for the degradation suppressingcontrol is established, the control routine is proceeded to step S13,where the ECU 13 prohibits the catalyst degradation suppressing control,that is, permits the execution of the deceleration F/C. Thereafter, thecurrent control routine is ended.

In contrast, if it is determined that precondition for the degradationsuppressing control is established, the control routine is proceeded tostep S14, where the ECU 13 determines whether the temperature (Tcat) ofthe catalyst 10 or 12 exceeds a predetermined determination temperatureA (for example, 800° C.), where the degradation of the catalystadvances. The temperature of the catalysts 10 and 12 can be estimated byreferring to, for example, an output from the exhaust gas temperaturesensor 9. If it is determined that the temperature (Tcat) of thecatalyst 10 or 12 does not exceed the predetermined determinationtemperature A, the control routine is proceeded to step S13, where theECU 13 prohibits the catalyst degradation suppressing control, andthereafter, ends the current control routine. In contrast, if determinedthat the temperature (Tcat) of the catalyst 10 or 12 exceeds thepredetermined determination temperature A, the control routine isproceeded to step S15, at which it is determined whether an integrationvalue (i.e., an integration Ga) of an intake air quantity (Ga) takeninto the internal combustion engine 1 after the fuel quantity increasingoperation of the internal combustion engine 1 exceeds a predeterminedquantity b. For example, an integration value of Ga, where oxygen inquantity enough to suppress generation of H₂S can be occluded in thecatalyst 10 or 12, is set as the predetermined quantity b. Theintegration Ga is calculated by integrating Ga acquired based on anoutput from the air flow sensor 6 according to a control routinedifferent from that shown in FIG. 2. When determined that theintegration Ga is the predetermined quantity b or smaller, the controlroutine is proceeded to step S13, where the ECU 13 prohibits thecatalyst degradation suppressing control, and thereafter, ends thecurrent control routine. In contrast, if it is determined that theintegration Ga is greater than the predetermined quantity b, the controlroutine is proceeded to step S16, where the ECU 13 permits the catalystdegradation suppressing control, that is, prohibits the execution of theF/C. Thereafter, the ECU 13 ends the current control routine.

In the above-described manner, a period (i.e., an operation permittingperiod), during which the execution of the deceleration F/C ispermitted, is set until the integration Ga reaches the predeterminedquantity b after the fuel quantity increasing operation of the internalcombustion engine 1 by executing the control routine shown in FIG. 2, sothat the oxygen in quantity enough to suppress the generation of H₂S canbe occluded in the catalyst 10 or 12. A completion timing of theoperation permitting period may be determined based on a lapse of timeafter the fuel quantity increasing operation in addition to theintegration Ga.

FIG. 3 is a flowchart showing the first example of an air-fuel ratiocontrol routine executed by the ECU 13 for controlling the air-fuelratio. The control routine shown in FIG. 3 is executed repeatedly in apredetermined cycle during the operation of the internal combustionengine 1. Here, the same processing in FIG. 3 as that in FIG. 2 isdesignated by the same reference numeral, and therefore, its explanationwill be omitted.

In the air-fuel ratio control routine shown in FIG. 3, the ECU 13 firstdetermines in step S21 whether a fuel quantity increasing condition,under which fuel to be supplied to the internal combustion engine 1 isincreased, is established. The fuel quantity increasing condition isestablished when, for example, output enhancement is required for theinternal combustion engine 1. If it is determined that the fuel quantityincreasing condition is not established, the control routine isproceeded to step S22, where the ECU 13 acquires the integration value(i.e., the integration GaS) of the quantity of intake air taken into theinternal combustion engine 1 during a stoichiometric operation, in whichthe air-fuel ratio is controlled to be a stoichiometric air-fuel ratio,after the fuel quantity increasing operation of the internal combustionengine 1. The integration GaS can be acquired by integrating outputsfrom the air flow sensor 6 after, for example, the internal combustionengine 1 transits from the fuel quantity increasing operation to thestoichiometric operation.

In next step S23, it is determined whether the internal combustionengine 1 is in a deceleration state. It is determined whether theinternal combustion engine 1 is in a deceleration state, for example,based on whether an opening degree (TA) of the throttle valve 7 issmaller than a predetermined opening degree C. Here, an opening degreeslightly greater than an opening degree during an idle operation of theinternal combustion engine 1, for example, is set as the predeterminedopening degree C. If it is determined that the internal combustionengine 1 is in a deceleration state, the control routine is proceeded tostep S11, where the ECU 13 determines whether the deceleration F/Ccondition is established. If it is determined that the deceleration F/Ccondition is established, the control routine is proceeded to step S24,where the ECU 13 determines whether a catalyst degradation suppressingcondition is established. Here, it is determined that the catalystdegradation suppressing condition is established when, for example, thethrottle valve 7 is normal and the temperature Tcat is higher than thepredetermined temperature A. If it is determined that the catalystdegradation suppressing condition is established, the control routine isproceeded to step S25, where the ECU 13 determines whether a flag XPW,for determining whether the fuel quantity increasing operation has beencarried out in the internal combustion engine 1, is 1 which indicatesthat the fuel quantity increasing operation has been carried out. If itis determined that the flag XPW is 1, the control routine is proceededto step S26, where it is determined whether the integration GaS is lessthan the predetermined quantity b. If it is determined that theintegration GaS is less than the predetermined quantity b, the controlroutine is proceeded to step S27, where the integration value of the Ga(i.e., an integration GaL) during operation in the lean air-fuel ratioin the internal combustion engine 1 after a fuel quantity increasingoperation is acquired. The integration GaL can be acquired byintegrating the Ga during, for example, the operation in the leanair-fuel ratio of the internal combustion engine 1.

In next step S28, the ECU 13 determines whether the integration GaL isless than a predetermined quantity d. For example, an integrating airintake quantity during the lean operation of the internal combustionengine 1, by which the oxygen in quantity enough to suppress thegeneration of H₂S can be occluded in the exhaust gas purifying catalyst10, is set as the predetermined quantity d. If it is determined that theintegration GaL is less than the predetermined quantity d, the controlroutine is proceeded to step S29, where the ECU 13 instructs a leancontrol to set the lean air-fuel ratio in the internal combustion engine1. Thereafter, the current control routine is ended.

If it is determined in step S24 that the catalyst degradationsuppressing condition is not established, the control routine isproceeded to step S30, where the ECU 13 executes the F/C. Thereafter,the current control routine is ended. If it is determined in step S26that the integration GaS is not less than the predetermined quantity bor it is determined in step S28 that the integration GaL is not lessthan a predetermined quantity d, the control routine is proceeded tostep S31, where the ECU 13 substitutes 0 into XPW. In next step S32, theECU 13 instructs the stoichiometric control for controlling the air-fuelratio in the internal combustion engine 1 to be the stoichiometricair-fuel ratio. Thereafter, the current control routine is ended. Whendetermined in step S23 that the internal combustion engine 1 is not in adeceleration state, determined in step S11 that the deceleration F/Ccondition is not established, or determined in step S25 that the XPW isnot 1, the control routine is proceeded to step S32, where the ECU 13instructs the internal combustion engine 1 to perform the stoichiometriccontrol. Thereafter, the current control routine is ended. If it isdetermined in step S21 that the fuel quantity increasing operationcondition of the internal combustion engine 1 is established, thecontrol routine is proceeded to step S33, where the ECU 13 substitutes 1into XPW. In next step S34, the ECU 13 initializes the integrations GaSand GaL by substituting 0 into them. In next step S35, the ECU 13instructs the fuel quantity increasing control so as to increase aquantity of fuel to be supplied to the internal combustion engine 1.Thereafter, the current control routine is ended.

In this manner, the lean control is instructed only when the internalcombustion engine 1 is in a deceleration state and the catalystdegradation suppressing condition is established in the air-fuel ratiocontrol routine shown in FIG. 3, thereby suppressing the degradation ofeach of the catalysts 10 and 12. Furthermore, a period during which thelean control is instructed (i.e., the lean control period) is ended bysubstituting 0 into the XPW when the integration GaL or GaS becomes thepredetermined quantity or more, and thereafter, the operation isperformed at the stoichiometric air-fuel ratio, thereby furthersuppressing the degradation of each of the catalysts 10 and 12. Here,the completion timing of the lean control period may be determined byutilizing a period during which the internal combustion engine 1 isoperated at the lean air-fuel ratio after the quantity increasingoperation in place of the integration GaL, and a period during which theinternal combustion engine 1 is operated at the stoichiometric air-fuelratio after the quantity increasing operation in place of theintegration GaS.

FIG. 4 is a flowchart showing the second example of an air-fuel ratiocontrol routine executed by the ECU 13. The air-fuel ratio controlroutine shown in FIG. 4 is different from the control routine shown inFIG. 3 in that a period, during which an air-fuel ratio in the internalcombustion engine 1 is controlled to be a lean air-fuel ratio, ischanged according to the integration GaS. The control routine shown inFIG. 4 is executed repeatedly in a predetermined cycle during operationof the internal combustion engine 1. Here, the same processing in FIG. 4as that in FIG. 3 is designated by the same reference numeral, andtherefore, its explanation will be omitted.

In the air-fuel ratio control routine shown in FIG. 4, the ECU 13 firstdetermines in step S21 whether a fuel quantity increasing condition ofthe internal combustion engine 1 is established. If it is determinedthat the fuel quantity increasing condition is not established, thecontrol routine is proceeded to step S22, where the ECU 13 acquires anintegration GaS. In next step S23, the ECU 13 determines whether theinternal combustion engine 1 is in a deceleration state. If it isdetermined that the internal combustion engine 1 is in a decelerationstate, the control routine is proceeded to step S11, where the ECU 13determines whether a deceleration F/C condition of the internalcombustion engine 1 is established. If it is determined that thedeceleration F/C condition is established, the control routine isproceeded to step S24, where the ECU 13 determines whether a catalystdegradation suppressing condition is established. Here, if it isdetermined that the catalyst degradation suppressing condition isestablished, the control routine is proceeded to step S25, where the ECU13 determines whether the flag XPW is 1.

If it is determined that the flag XPW is 1, the control routine isproceeded to step S41, where a determination integration value E fordetermining the completion of a lean control period in the internalcombustion engine 1 according to the integration GaS is acquired. Thedetermination integration value E can be acquired by, for example,referring to a curve E1 in a map shown in FIG. 5. Since oxygen iscontained in the exhaust gas during a stoichiometric operation of theinternal combustion engine 1, the oxygen is gradually occluded incatalysts 10 and 12 also during the stoichiometric operation. As aconsequence, for example, when the integration GaS is great (that is,the stoichiometric operation period in the internal combustion engine 1is long), the oxygen in quantity enough to suppress generation of H₂Scan be occluded in the catalysts 10 and 12 even if the lean controlperiod during the deceleration of the internal combustion engine 1becomes short.

In next step S42, an integration GaI serving as an integration value ofGa taken into the internal combustion engine 1 after the air-fuel ratioin the internal combustion engine 1 is controlled to be a lean air-fuelratio is acquired. The integration GaI can be acquired, for example, byintegrating the Ga during the operation at the lean air-fuel ratio inthe internal combustion engine 1 after the fuel quantity increasingoperation. In next step S43, the ECU 13 determines whether theintegration GaI is less than the determination integration value E. Ifit is determined that the integration GaI is less than the determinationintegration value E, the control routine is proceeded to step S29, wherethe ECU 13 instructs the internal combustion engine 1 to perform a leancontrol. Thereafter, the current control routine is ended.

If it is determined in step S24 that the catalyst degradationsuppressing condition is not established, the control routine isproceeded to step S30, where the ECU 13 executes the F/C. Thereafter,the current control routine is ended. If it is determined in step S43that the integration GaI is not less than the determination integrationvalue E, the control routine is proceeded to step S31, where the ECU 13substitutes 0 into the XPW. In next step S33, the ECU 13 instructs theinternal combustion engine 1 to perform a stoichiometric control.Thereafter, the current control routine is ended. When determined instep S23 that the internal combustion engine 1 is not in a decelerationstate, determined in step S11 that the deceleration F/C condition is notestablished, or determined in step S25 that the XPW is not 1, thecontrol routine is proceeded to step S32. Thereafter, the sameprocessing as that shown in FIG. 4 is performed, and then, the currentcontrol routine is ended.

If it is determined in step S21 that the fuel quantity increasingoperation condition in the internal combustion engine 1 is established,the control routine is proceeded to step S33, where the ECU 13substitutes 1 into the XPW. In next step S44, the ECU 13 initializes theintegrations GaS and GaI by substituting 0 into them. In next step S35,the ECU 13 instructs the internal combustion engine 1 to perform thefuel quantity increasing operation. Thereafter, the current controlroutine is ended.

As described above, in the control routine shown in FIG. 4, thedetermination integration value E is adjusted according to theintegration GaS, so that the lean control period is properly adjusted.If the integration GaS is great, the determination integration value Eis reduced, and further, the lean control period is shortened.Therefore, it is possible to suppress the degradation of each of thecatalysts 10 and 12. In contrast, if the integration GaS is small, thedetermination integration value E is increased, and further, the leancontrol period is prolonged. The lean control period is prolonged inthis manner, so that the oxygen in quantity enough to suppress thegeneration of H₂S can be occluded in the catalyst 10 or 12.

FIGS. 6 and 7 are flowcharts showing the third example of an air-fuelratio control routine executed by the ECU 13. The air-fuel ratio controlroutine shown in FIG. 6 is different from the air-fuel ratio controlroutine in the other examples in that an integration flow rate of oxygenflowing into catalysts 10 and 12 after a fuel quantity increasingoperation is estimated based on Ga, and thus, that a lean control periodis changed according to the integration flow rate of the flowing oxygen.The control routine shown in FIG. 6 is executed repeatedly in apredetermined cycle during operation of the internal combustion engine1. Here, the same processing in FIGS. 6 and 7 as that in FIGS. 3 and 4is designated by the same reference numeral, and therefore, itsexplanation will be omitted.

In the control routine shown in FIG. 6, the ECU 13 first determines instep S21 whether a fuel quantity increasing condition in the internalcombustion engine 1 is established. If it is determined that the fuelquantity increasing condition is not established, the control routine isproceeded to step S23, where the ECU 13 determines whether the internalcombustion engine 1 is in a deceleration state. If it is determined thatthe internal combustion engine 1 is in a deceleration state, the controlroutine is proceeded to step S11, where the ECU 13 determines whether adeceleration F/C condition of the internal combustion engine 1 isestablished. If it is determined that the deceleration F/C condition isestablished, the control routine is proceeded to step S24, where the ECU13 determines whether a catalyst degradation suppressing condition isestablished. Here, if it is determined that the catalyst degradationsuppressing condition is established, the control routine is proceededto step S61, where a total value (TO2) of the integration flow rate ofoxygen flowing into the catalysts 10 and 12 after the fuel quantityincreasing operation is calculated. The TO2 is obtained by summing anintegration flow rate (O2L) of oxygen flowing into the catalysts 10 and12 during a lean operation of the internal combustion engine 1 acquiredin step S63, an integration flow rate (O2FC) of oxygen flowing into thecatalysts 10 and 12 during the F/C, acquired in step S65, and anintegration flow rate (O2S) of oxygen flowing into the catalysts 10 and12 during a stoichiometric operation of the internal combustion engine 1acquired in step S66. The values O2S, O2FC, and O2L are stored in a RAMin the ECU 13, and the previous values are held until new values aresubstituted therein.

In next step S25, the ECU 13 determines whether a flag XPW is 1. If itis determined that the XPW is 1, the control routine is proceeded tostep S62, where the ECU 13 determines whether the TO2 is less than adetermination oxygen quantity F. As the determination oxygen quantity F,for example, an oxygen quantity enough to suppress generation of H₂S isset. If it is determined that the TO2 is less than the determinationoxygen quantity F, the control routine is proceeded to step S27, wherethe ECU 13 acquires the integration GaL. In next step S63, the ECU 13acquires the O2L based on the integration GaL. The flow rate of oxygenflowing into the catalysts 10 and 12 can be estimated based on the Ga.Thus, the integration flow rate of oxygen flowing into the catalysts 10and 12 is estimated based on the integration Ga by using a map shown inFIG. 8. Since the quantity of fuel to be supplied to the internalcombustion engine 1 varies according to whether the internal combustionengine 1 is operated under which state of the lean air-fuel ratio, theF/C, or the stoichiometric air-fuel ratio, the concentration of oxygenin the exhaust gas from the internal combustion engine 1 is variedaccording to the operation state of the internal combustion engine 1. Inview of this, in order to acquire the integration flow rate of oxygenaccording to the operation state, lines corresponding to theintegrations Ga in the respective operation states are shown in the mapshown in FIG. 8. When the O2L is acquired based on the integration GaL,a line L shown in FIG. 8 is used. For example, when the integration GaLis GaL1 shown in FIG. 8, the value O2L becomes O2L1 shown in FIG. 8. TheECU 13 functions as oxygen integration flow rate acquiring means byexecuting the above-described processing. In subsequent step S29, theECU 13 instructs the internal combustion engine 1 to perform the leancontrol. Thereafter, the current control routine is ended.

If it is determined in step S24 that the catalyst degradationsuppressing condition is not established, the control routine isproceeded to step S64, where the ECU 13 acquires an integration GaFC asan integration value of Ga at the time of the F/C. The integration GaFCis calculated by, for example, integrating the Ga acquired from the airflow sensor 6 at the time of the F/C. In subsequent step S65, the ECU 13acquires an O2FC based on the integration GaFC with reference to FIG. 8.When acquiring the O2FC, a line FC shown in FIG. 8 is used. In next stepS30, the ECU 13 executes the F/C. Thereafter, the current controlroutine is ended.

When determined in step S23 that the internal combustion engine 1 is notin the deceleration state, the control routine is proceeded to step S22,where the ECU 13 acquires the integration GaS. In next step S66, the ECU13 acquires an O2S based on the integration GaS referring to the mapshown in FIG. 8. When acquiring the O2S, a line S shown in FIG. 8 isused. In next step S32, the ECU 13 instructs the internal combustionengine 1 to perform the stoichiometric control. Thereafter, the currentcontrol routine is ended. If it is determined in step S62 that the TO2is not less than the determination oxygen quantity F, the controlroutine is proceeded to step S31, where the ECU 13 substitutes 0 intothe XPW. At subsequent step S32, the ECU 13 instructs the internalcombustion engine to perform a stoichiometric control. Thereafter, thecurrent control routine is ended.

If it is determined in step S21 that the fuel quantity increasingoperation condition is established, the control routine is proceeded tostep S33 shown in FIG. 7, where the ECU 13 substitutes 1 into the XPW.In next step S67, the ECU 13 substitutes 0 into the integrations GaS,GaFC, GaL, O2S, O2L and O2FC, and thus, initializes these values. Innext step S35, the ECU 13 instructs the internal combustion engine 1 toperform the fuel quantity increasing control. Thereafter, the currentcontrol routine is ended.

As described above, the completion of the period, during which theair-fuel ratio in the internal combustion engine 1 is controlled to be alean air-fuel ratio, is determined based on the TO2 containing the O2FCin the control routine shown in FIG. 6. As a consequence, for example,when the F/C is executed for a long period after, the fuel quantityincreasing operation, the O2FC becomes great, and thus, the lean controlperiod becomes shorter. In contrast, the O2FC becomes small when the F/Cis executed for a short period, and thus, the lean control periodbecomes longer. Since the concentration of the oxygen contained in theexhaust gas during the F/C becomes substantially the same as that of theair, a great quantity of oxygen can be occluded in the catalysts 10 and12. However, when the fuel cutting period is short, there is apossibility that the oxygen in quantity enough to suppress thegeneration of H₂S cannot be occluded in the catalyst 10 or 12. Thus,only in such a case, the air-fuel ratio in the internal combustionengine 1 is controlled to be a lean air-fuel ratio, so that the oxygenin quantity enough to suppress the generation of H₂S is occluded in thecatalyst 10 or 12.

FIG. 9 is a timing chart showing one example of changes in time of theoxygen occlusion quantity in the catalysts 10 and 12 and an output fromthe oxygen concentration sensor 11 in the case where the air-fuel ratiocontrol routine shown in FIG. 6 is executed. Comparative examples arealso shown in FIG. 9, in which a time change is shown in the case wherean air-fuel ratio is made lean only for a certain period after a fuelquantity increasing operation. When an increase in fuel quantity isinstructed to the internal combustion engine 1 at a timing t1 in FIG. 9,the air-fuel ratio in the internal combustion engine 1 becomes rich.Consequently, the air-fuel ratio of the exhaust gas becomes rich, andfirst, the oxygen occlusion quantity in the start catalyst 10 is startedto be decreased. When the oxygen occlusion quantity in the startcatalyst 10 becomes 0 at a timing t2 in FIG. 9, the output from theoxygen concentration sensor 11 is changed onto a rich side, and thus,the rich exhaust gas is started to flow into the three way catalyst 12.Therefore, the oxygen occlusion quantity in the three way catalyst 12 isstarted to be decreased. Thereafter, the oxygen occlusion quantity inthe three way catalyst 12 becomes 0 at a timing t3.

Since the air-fuel ratio in the internal combustion engine 1 iscontrolled to be a lean air-fuel ratio upon completion of the fuelquantity increasing operation at a timing t4 in FIG. 9, oxygen occlusionis first started in the start catalyst 10. Upon completion of the oxygenocclusion in the start catalyst 10 up to a maximum oxygen occlusionquantity at a timing t5, the oxygen occlusion is started in the threeway catalyst 12. According to the present invention, thereafter, theair-fuel ratio in the internal combustion engine 1 is controlled to be alean air-fuel ratio till a timing t7, so that the oxygen is occluded inthe three way catalyst 12. In contrast, when the air-fuel ratio iscontrolled to be a lean air-fuel ratio only for a certain period afterthe fuel quantity increasing operation, the air-fuel ratio in theinternal combustion engine 1 is controlled to be a lean air-fuel ratioonly till a timing t6, so that the oxygen can be sufficiently occludedin the start catalyst 10 but little oxygen can be occluded in the threeway catalyst 12. In this manner, the oxygen can be sufficiently occludedin the catalysts 10 and 12 by executing the control routine shown inFIG. 6, thus suppressing the generation of H₂S after the stoppage of theinternal combustion engine 1.

Second Embodiment

An air-fuel ratio control apparatus in the second embodiment accordingto the present invention will be described below with reference to FIGS.10 and 11. Here, the same members in FIG. 10 as those in FIG. 1 aredesignated by the same reference numerals, and therefore, theexplanation thereof will be omitted.

The second embodiment shown in FIG. 10 is different from the firstembodiment in that there is provided a secondary air supplier 15 servingas secondary air supplying means for supplying secondary air, and thesecondary air supplier 15 and the exhaust passage 4 are connected eachother via a secondary air passage 16 at a portion between the startcatalyst 10 and the three way catalyst 12. In the secondary air passage16 is disposed a supply valve 17 serving as valve means for switchingthe connection to or disconnection from the secondary air passage 16 byopening and closing so as to supply the secondary air to the exhaustpassage 4. In other words, the supply valve 17 is capable of switchingthe connection state of the secondary air passage 16, and the secondaryair can be supplied to the exhaust passage 4 by making the supply valve17 open. Here, the secondary air supplier 15 supplies the secondary airto the exhaust passage 4 via, for example, an air pump or an airsuction.

The ECU 13 controls the operation of the supply valve 17. FIG. 11 is aflowchart showing a secondary air supply control routine to be executedby the ECU 13 in order to supply the secondary air to the exhaustpassage 4. The control routine shown in FIG. 11 is executed repeatedlyin a predetermined cycle during the operation of the internal combustionengine. Here, the same processing in FIG. 11 as that in FIG. 3 isdesignated by the same reference numeral, and therefore, its explanationwill be omitted. The ECU 13 functions as valve control means byexecuting the control routine shown in FIG. 11.

In the secondary air supply control routine shown in FIG. 11, the ECU 13first determines in step S101 whether the internal combustion engine 1is in the deceleration state or an idle operation state. The operationstate of the internal combustion engine 1 can be estimated by referringto, for example, the opening degree of the throttle valve 7. If it isdetermined that the internal combustion engine 1 is in the decelerationstate or the idle operation state, the control routine is proceeded tostep S11, where the ECU 13 determines whether the deceleration F/Ccondition is established. If it is determined that the deceleration F/Ccondition is not established, the control routine is proceeded to stepS15, where the ECU 13 determines whether an integration GaS is less thana predetermined quantity B. If it is determined that the integration GaSis less than the predetermined quantity B, the control routine isproceeded to step S102, where the ECU 13 determines whether an open flagindicating an open state of the supply valve 17 is ON. If it isdetermined that the open flag is not ON, the control routine isproceeded to step S103, where the ECU 13 opens the supply valve 17 so asto start supplying the secondary air. In next step S104, the ECU 13turns ON the open flag. In subsequent step S105, the ECU 13 starts toactuate a timer. This timer is used to determine whether a predeterminedperiod of time is elapsed after the supply valve 17 is opened.Thereafter, the current control routine is ended.

In contrast, if it is determined in step S102 that the open flag is ON,the control routine is proceeded to step S106, where the ECU 13determines whether a predetermined determination period G of time iselapsed after the timer is started to be actuated. Here, as thedetermination period G, for example, a period during which oxygen inquantity enough to suppress generation of H₂S can be occluded in thethree way catalyst 12 by the supply of the secondary air is set. If itis determined that the determination period G is not elapsed, thecurrent control routine is ended. In contrast, if it is determined thatthe determination period G is elapsed, the control routine is proceededto step S107, where the supply of the secondary air is stopped byclosing the supply valve 17. In next step S108, the ECU 13 resets theflag, and thereafter, the current control routine is ended. Also if thedetermination result is negative in step S101, the determination resultis affirmative in step S11, or the determination result is negative instep S15, the control routine is proceeded to step S107, where the ECU13 closes the supply valve 17. Subsequently, the flag is reset in stepS108, and thereafter, the current control routine is ended.

Since the control routine shown in FIG. 11 is executed in theabove-described manner, the secondary air is supplied to the catalyst 12for the predetermined supply period of time (that is, a period until thedetermination period G is elapsed) if no deceleration F/C is executed,so that the oxygen in quantity enough to suppress the generation of H₂Scan be occluded in the catalyst 12. Moreover, the secondary air issupplied downstream from the start catalyst 10, thereby suppressing thedegradation of the start catalyst 10.

The determination period G may be changed according to a period duringwhich the F/C is executed at the time of the deceleration after the fuelquantity increasing operation (that is, a fuel cutting period). When thefuel cutting period is long, it is considered that the oxygen inquantity enough to suppress the generation of H₂S can be occluded in thethree way catalyst 12 by supplying a small quantity of secondary air.Therefore, the determination period G used in step S106 shown in FIG. 11may be set with reference to, for example, a map shown in FIG. 12. FIG.12 shows one example of the relationship between the fuel cutting periodand the determination period G. In FIG. 12, the longer the fuel cuttingperiod, the shorter the determination period.

In the above-described manner, the oxygen in quantity enough to suppressthe generation of H₂S can be occluded in the three way catalyst 12 bychanging the determination period G according to the fuel cuttingperiod, and further, the degradation of the three way catalyst 12 can besuppressed by properly adjusting the quantity of secondary air to besupplied to the three way catalyst 12.

The second embodiment can be applied to the internal combustion enginehaving a plurality of catalysts disposed in the exhaust passage. Asshown in, for example, FIG. 13A, when the three way catalyst 12 isdivided into two catalysts 12 b and 12 c inside a casing 12 a inarrangement, the casing 12 a and the secondary air supplier 15 areconnected to each other in such a manner as to supply the secondary airto a portion between the two catalysts 12 b and 12 c, thus producing asimilar effect. Otherwise, as shown in FIG. 13B, when three catalystsare disposed in the exhaust passage 4 by additionally disposing anexhaust gas purifying catalyst 18 downstream from the catalyst 12, thesecondary air supplier 15 is connected between the respective twocatalysts, thus producing a similar effect. Here, when the threecatalysts are disposed in the exhaust passage 4 in the above-describedmanner, the secondary air passage 16 may be connected both between thecatalysts 10 and 12 and between the catalysts 12 and 18, or thesecondary air passage 16 may be connected either between the catalysts10 and 12 or between the catalysts 12 and 18. Also in an internalcombustion engine having four or more catalysts disposed in the exhaustpassage, a similar effect can be produced by disposing the secondary airpassage 16 on at least one portion between the catalysts.

The present invention is not limited to the above-described embodiments,but may be embodied in various modes. For example, the integrationstarting timing of the integration Ga used in the control routineaccording to the present invention is not limited to the timing afterthe completion of the fuel quantity increasing operation, but theintegration may be started after the O₂ sensor 11 outputs a signal on alean side. FIGS. 14 to 16 show a control routine for determining theintegration starting timing by the O₂ sensor 11 in the above-describedmanner.

FIG. 14 is a flowchart showing a modification of the catalystdegradation suppressing control routine shown in FIG. 2. The controlroutine shown in FIG. 14 is also executed repeatedly in a predeterminedcycle during the operation of the internal combustion engine 1. Here,the same processing in FIG. 14 as that in FIG. 2 is designated by thesame reference numeral, and therefore, its explanation will be omitted.

In the control routine shown in FIG. 14, the ECU 13 performs the sameprocessing as that shown in FIG. 2 until the ECU 13 determines whetherthe temperature Tcat exceeds the determination temperature A (till stepS14). If it is determined that the temperature Tcat exceeds thedetermination temperature A, the control routine is proceeded to stepS201, where the ECU 13 determines whether a signal output from the O₂sensor 11 is changed onto a lean side after the fuel quantity increasingoperation of the internal combustion engine 1. If it is determined thatthe signal is not changed onto the lean side, the control routine isproceeded to step S13, where the ECU 13 prohibits the catalystdegradation suppressing control, and then, the current control routineis ended. In contrast, if it is determined that the signal is changedonto the lean side, the control routine is proceeded to step S15, wherethe same processing as that shown in FIG. 2 is performed, and then, thecurrent control routine is ended.

In this manner, according to the control routine shown in FIG. 14, it isdetermined whether the integration Ga exceeds the predetermined quantityb after the signal output from the O₂ sensor 11 is changed onto the leanside in step S201, that is, after the air-fuel ratio of the exhaust gasflowing into the catalyst 12 is changed to the lean air-fuel ratio.Consequently, the completion timing of the operation permitting periodis determined by using the integration value of the intake air quantitytaken into the internal combustion engine 1 after the signal output fromthe O₂ sensor 11 is changed onto the lean side. The completion timing ofthe operation permitting period in the control routine shown in FIG. 3may be determined based on the lapse of time after the signal outputfrom the O₂ sensor 11 is changed onto the lean side in addition to theintegration Ga.

FIG. 15 is a flowchart showing a modification of the air-fuel ratiocontrol routine shown in FIG. 3. The control routine shown in FIG. 15 isalso executed repeatedly in a predetermined cycle during the operationof the internal combustion engine 1. Here, the same processing in FIG.15 as that in FIGS. 4 and 14 is designated by the same referencenumeral, and therefore, its explanation will be omitted.

In the control routine shown in FIG. 15, the ECU 13 performs the sameprocessing as that shown in FIG. 3 until the ECU 13 determines whetherthe fuel quantity increasing condition is established (till step S21).If it is determined that the fuel quantity increasing condition isestablished, the control routine is proceeded to step S201, where theECU 13 determines whether a signal output from the O₂ sensor 11 ischanged onto a lean side after the fuel quantity increasing operation.If it is determined that the signal is changed onto the lean side, thecontrol routine is proceeded to step S22, where the ECU 13 acquires theintegration GaS. After the same processing as that shown in FIG. 3 isperformed, the current control routine is ended. In contrast, if it isdetermined that the signal is not changed onto the lean side, thecontrol routine is proceeded to step S23. After the same processing asthat shown in FIG. 3 is performed, the current control routine is ended.

In this manner, according to the control routine shown in FIG. 15, theintegration GaS is acquired after the signal output from the O₂ sensor11 is changed onto the lean side. Consequently, the completion timing ofthe lean control period is determined based on the integration GaS afterthe oxygen is started to be supplied to the catalyst 12, and thereforeit can be more accurately determined whether the oxygen in quantityenough to suppress the generation of H₂S can be occluded in the catalyst12. The completion timing of the lean control period in the controlroutine shown in FIG. 15 may be determined based on the lapse of timeafter the signal output from the O₂ sensor 11 is changed onto the leanside in addition to the integration Ga.

FIG. 16 is a flowchart showing a modification of the air-fuel ratiocontrol routine shown in FIG. 6. The control routine shown in FIG. 16 isalso executed repeatedly in a predetermined cycle during the operationof the internal combustion engine 1. Here, the same processing in FIG.16 as that in FIGS. 6 and 14 is designated by the same referencenumeral, and therefore, its explanation will be omitted.

The control routine shown in FIG. 16 is different from that shown inFIG. 6 in thin step S201 is executed if the ECU 13 determines that theresult is affirmative in step S62, step S202 is executed if the ECU 13determines that the result is negative in step S24, and step S203 isexecuted if the ECU 13 determines that the result is negative in stepS23 or step S11. In step S201, the ECU 13 determines whether the signaloutput from the O₂ sensor 11 is changed onto a lean side after the fuelquantity increasing operation. If it is determined that the signal ischanged onto the lean side, the control routine is proceeded to stepS27, and in contrast, if it is determined that the signal is not changedonto the lean side, the control routine is proceeded to step S29. Alsoin steps S202 and S203, the ECU 13 determines whether the signal outputfrom the O₂ sensor 11 is changed onto the lean side after the fuelquantity increasing operation. If the ECU 13 determines that the resultis affirmative in step S202, the control routine is proceeded to stepS64, and in contrast, if the ECU 13 determines that the result isnegative, the control routine is proceeded to step S30. If the ECU 13determines that the result is affirmative in step S203, the controlroutine is proceeded to step S22, and in contrast, if the ECU 13determines that the result is negative, the control routine is proceededto step S32.

In this manner, the quantity of oxygen flowing into the catalyst 12 canbe more accurately calculated by acquiring the integration Ga after thesignal output from the O₂ sensor 11 is changed onto the lean side afterthe fuel quantity increasing operation. The completion timing of thelean control period in the control routine shown in FIG. 16 may also bedetermined based on the lapse of time after the signal output from theO₂ sensor 11 is changed onto the lean side in addition to theintegration Ga in the same manner as that shown in FIG. 15.

Furthermore, the integration starting timing of the integration GaS inthe control routine shown in FIG. 4 may also be started after the O₂sensor 11 outputs the signal on the lean side. In this case, since theGa of the period during which the oxygen is occluded in the catalyst 10is not integrated, the integration GaS becomes small. Consequently, ifthe integration starting timing of the integration GaS in the controlroutine shown in FIG. 4 is set after the O₂ sensor 11 outputs the signalon the lean side, the determination integration value E is acquired byusing a curve E2 in FIG. 5 in step S41.

As described above, in the control routines shown in FIGS. 14 to 16,after the signal output from the O₂ sensor 11 is changed onto the leanside, that is, after the air-fuel ratio of the exhaust gas flowing intothe catalyst 12 is changed to the lean air-fuel ratio, the calculationof the integration Ga is started, and thus, the completion timing isdetermined based on the integration Ga. Thus, it can be more accuratelydetermined whether the oxygen in quantity enough to suppress thegeneration of H₂S can be occluded in the catalyst 12.

As is described above, according to the present invention, it ispossible to suppress the generation of the catalyst exhaust gas odor,because the air-fuel ratio is controlled to be the lean air-fuel ratioaccording to the operation state at the time of the deceleration afterthe fuel quantity increasing operation of the internal combustion engineand the oxygen is occluded in the exhaust gas purifying catalyst.Additionally, the period, during which the air-fuel ratio is set to thelean air-fuel ratio, is properly adjusted, thus suppressing thedegradation of the exhaust gas purifying catalyst.

1. An air-fuel ratio control apparatus of an internal combustion engine comprising: a plurality of exhaust gas purifying catalysts disposed in an exhaust gas purifying catalyst casing, the exhaust gas purifying catalyst casing disposed in an exhaust passage of the internal combustion engine; secondary air supplying means for supplying secondary air through a secondary air passage to at least one portion of the exhaust gas purifying catalyst casing, the portion of the exhaust gas purifying catalyst casing being disposed between the plurality of exhaust gas purifying catalysts; valve means for switching connection to or disconnection from the secondary air passage; valve controlling means for controlling operation of the valve means; and fuel cutting means for cutting fuel to be supplied to the internal combustion engine at the time of deceleration of the internal combustion engine, wherein the valve controlling means sets a supplying period, during which the secondary air is supplied by switching the valve means to a connection state during a non-operating period of the fuel cutting means after completion of a fuel quantity increasing operation of the internal combustion engine.
 2. The air-fuel ratio control apparatus of the internal combustion engine according to claim 1, wherein the valve controlling means sets the supplying period to be shorter as a fuel cutting period by the fuel cutting means after the completion of the fuel quantity increasing operation is longer. 